In a known conventional communication system, a master (a transmission unit) and a plurality of communication terminals (slaves) are connected to a communication line (transmission line) such that communication is performed between the respective communication terminals and the master. In an example of this type of communication system, the master monitors the conditions of the respective communication terminals periodically, and when the condition of a communication terminal varies, the master transmits a signal to another communication terminal so that dealing with the condition variation (see Patent Documents 1 to 3, for example).
In this configuration, however, the communication terminals always communicate via the master. Because the master polls the communication terminals, a communication speed is slow. As a result, the system is unsuitable for transmitting information including comparatively large data amount, for example an analog quantity. Further, when the master breaks down or the like, the entire system stops, and therefore reliability as a communication system is poor.
In response to this problem, it has been proposed a communication system in which the communication terminals connected to the communication line are configured to communicate directly via peer to peer (P2P) communication, thereby increasing the communication speed so that information including a comparatively large data amount can be transmitted. This communication system typically employ a line power feeding method in which the master is used as a power supply apparatus and electric power is supplied from the single master to the plurality of communication terminals via the communication line.
Incidentally, in a communication system where the communication terminals directly communicate with each other, as described above, a plurality of terminals (the master and the communication terminals) are connected to the two-wire communication line. Therefore, input impedances of the respective terminals are connected in parallel via the communication line. Hence, in order to perform communication between the communication terminals using a voltage signal, the impedances of the terminals connected to the communication line must be increased. For example, the master serves as a power supply apparatus and therefore includes a smoothing capacitor or the like at an output stage thereof. As a result, the master exhibits comparatively low impedance. Therefore, a high impedance module (not shown) that exhibits high impedance relative to the voltage signal on the communication line must be connected between this type of terminal and the communication line so as to increase the input impedance relative to a signal component. If, for example, a constructor forgets to connect the high impedance module, the impedance of the communication line is to decrease, and as a result, it may be impossible to establish communication between the communication terminals.
Hence, when the communication system described above is introduced, it requires a great labor to examine onsite conditions and the like, and it leads to an increased load on a constructor. As a result, the communication system cannot be introduced easily.
A communication system may be configured to transmit a current signal over the communication line by configuring that connecting a transmission unit of the communication terminal to the communication line and varying a current flowing through the communication line from the master serving as the power supply apparatus. In this case, a receiving unit of the communication terminal is configured to receive a voltage signal obtained by converting the current signal transmitted from the transmission unit through a current/voltage conversion unit provided between the master and the communication line.
Hence, signal transmission from the communication terminal is performed through the current signal, and signal reception in the communication terminal is performed through the voltage signal obtained by converting the current signal through the current/voltage conversion unit. By employing a current signal as the signal transmitted from the communication terminal, communication can be performed even when the impedance of the communication line decreases. As a result, it can reduce the labor required to examine the onsite conditions and the like during introduction of the communication system, and it leads to a reduction in the load on the constructor. Therefore, the communication system can be introduced easily.